1. Field of the Invention
The present invention relates to a far ultraviolet dielectric multilayer film which reflects beams in the far ultraviolet region and transmits beams in other wavelength regions.
2. Description of the Prior Art
At present, with development of super LSI, attention in this technical field is directed to X-ray lithography, electron beam lithography and deep ultraviolet lithography. Particularly of interest is a far ultraviolet exposure method which is a kind of transfer lithography employing ultraviolet rays of wavelength ranging from 200 to 300 nm and which was developed on the basis of known techniques of photolithography. According to the method it is possible to transfer a very fine pattern less than 1.mu..
Printing on photoresist is carried out using a mirror illumination system in which an Xe-Hg lamp is used as a light source. The mirror used in the illumination system is prepared by vapor deposition of aluminum and magnesium fluoride. However, since the mirror has a high reflectance to such beam of light having a wavelength longer than 200 nm, an undue rise of temperature is caused in the silicon wafer, which results in reduction of accuracy of the transferred pattern.
As for a multilayer film which reflects effectively far ultraviolet beams of light in the wavelength region of 200 to 250 nm but transmits beams of light longer than 250 nm, there has been known one such film which comprises alternate high refractive index and low refractive index layers with thorium oxide being used in the high refractive index layer. Thorium oxide absorbs almost no beams of light in the far ultraviolet region and therefore it causes no reduction of reflection power. On the contrary, high refractive dielectric substances other than thorium oxide generally absorb far ultraviolet rays, thereby reducing reflection power. This is the reason why thorium oxide is used in the high refractive index layer of such known multilayer film. However, thorium oxide is radioactive and its use in such multilayer film involves some problems. Therefore, it is desirable that use of thorium oxide be avoided.
To achieve high reflectance to far ultraviolet beams of light without any problem it has also been proposed to use a non-absorptive intermediate refractive index film (ThF.sub.4, Al.sub.2 O.sub.3, BaF.sub.2) instead of the above mentioned high refractive dielectric film. In this case, the multilayer film is composed of alternate intermediate refractive index and low refractive index layers. However, with this arrangement of multilayer film, a satisfactorily high reflection power can be obtained only when a large number of layers such as 20-30 layers are used. This makes the manufacture of such a multilayer film very difficult. In addition, the reflection band width becomes narrowed. A desirable width of reflection band may be attained only with much difficulty.